MOS transistors are commonly used in analog integrated circuits as switches. That is, the circuit designs use these devices in two states, namely "on" and "off". In order to turn the devices on, the gate-source voltage of the transistor must be sufficiently high to ensure that the device is fully on, and hence that the source-drain impedance is low enough to provide adequate performance. However, in a circuit with a relatively low supply voltage, for example in a battery-powered device, in which it is considered desirable to reduce the number and size of batteries which are used, it can be difficult to provide a sufficiently high gate-source voltage. This can advantageously be achieved by providing a gate voltage which is higher than the positive supply voltage of the circuit.
EP-A-0589123 discloses a voltage doubling circuit, which provides an output voltage which is in phase with an input clock signal, but which relies on a diode to charge a capacitor. The presence of the diode results in a voltage drop across the diode.